Abstract

We have combined H-alpha and UBV measurements of 210 nearby Sa-Irr
galaxies with new photometric synthesis models to reanalyze the past and
future star formation timescales in disks. The integrated
photoionization rates and colors of disks are best fitted by a stellar
initial mass function (IMF) which is enriched in massive stars by a
factor of 2-3 relative to the Scalo solar neighborhood IMF. We have used
published surface photometry of spiral galaxies to analyze the star
formation histories of disks independent of their bulge properties. The
ratio of the current star formation rate (SFR) to the average past rate
increases from of order 0.01 in Sa galaxies to 1 in Sc-Irr disks. This
confirms that the pronounced change in the photometric properties of
spiral galaxies along the Hubble sequence is predominantly due to
changes in the star formation histories of disks, and only secondarily
to changes in the bulge/disk ratio. A comparison of current SFRs and gas
masses of the sample yields median timescales for gas consumption of
approximately 3 Gyr, in the absence of stellar recycling. However, a
proper time-dependent treatment of the gas return from stars shows that
recycling extends the gas lifetimes of disks by factors of 1.5-4 for
typical disk parameters. Consequently the current SFRs in many (but not
all) disks can be sustained for periods comparable to the Hubble time.

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